WP Filesystem changes

Description

Just a few changes/enhancements/bugfixes for the Filesystem Abstraction classes.

Bring the FTP Constants inline with find_folder() - Moves all the processing for the various folders to a single function, much easier to handle

Changes the folder locations from WP_Filesystem to -always- return a trailing slash, This is for maximum compatibility with all host setups, ie. abspath == ftp foot == /, In the current setup, it can end up using a path such as which causes some problems with some setups

Lots of formatting cleanups

Return a standardised chmod return values for all classes, May need further testing, But given each class was returning something different, and Core isnt using it, I'm pretty certain its a safe change

return false on attempting to delete a empty path - If a bug occurs, and something asks to delete "null" or "" then prevent it from deleting, some take this to mean delete "." or "/"

(FTP) Better support for listing for a single file in a directory (ie. /wordpress/.maintanence)